In recent years, ingots (referred to also as strands) of steel or various kinds of alloys or the like are mass-produced generally by using a so-called “continuous casting method” which includes the steps of continuously injecting a molten alloy or the like into a water-cooled mold and gradually drawing out solidified ingots out of the mold.
There is a history that practical use of continuous casting originated with continuous casting machines for billets and blooms and thereafter continuous casting of slabs having larger cross-sectional areas has increased because of strong demands for energy saving and productivity improvement.
In order to obtain high-quality ingots with less non-metallic inclusions and less component segregation by the above-described continuous casting, it is important to stir the molten metal in the middle of solidification as required. Also, stirring the molten metal in slabs which are larger in cross-sectional area and moreover larger in length-to-width ratio of the cross-sectional area (e.g., the ratio of the length of the longer side wall to the length of the shorter side wall being 5 or more) would be highly liable to such problems as occurrence of center segregation, center cross-sectional cracks as well as degradation of machinability, unlike the case of strands which are small in cross-sectional area and moreover nearly square in cross-sectional shape such as blooms or billets. For this reason, there has been a need for stirring the molten metal as required.
As a countermeasure of the technique of molten metal stirring in continuous casting, a method in which, for example, an electromagnetic stirrer is provided near a cooling mold or on a back face of a cooling mold and molten metal is stirred by utilizing electromagnetic force, is known. However, since the electromagnetic stirrers are quite expensive devices, there has been a demand for inexpensive devices substitutable for these electromagnetic stirrers, to stir molten metal in the cooling mold.
As a solution given by the above-described inexpensive devices, there is proposed such methods as Patent Documents 1 to 6 for blooms or billets having nearly square cross-sectional shapes.
Patent Document 1 discloses a method for generating a horizontal rotational flow in the molten metal within the mold by an arrangement that four discharge holes are provided in rotational symmetry in a lower portion of a submerged nozzle in a slant direction, more preferably an angle of (45±10)°, to a square mold plane. Although this method improved the quality of strands of blooms or billets, the extent of the effect was not sufficient. Therefore, Patent Document 2 improves Patent Document 1 and proposes a method for generating a horizontal rotational flow in the molten metal within the mold to stir the molten metal within the mold by inclining the direction of the molten metal discharged from four discharge holes so as to be along directions of constant angles relative to each mold surface of a square mold instead of being in rotational symmetry, i.e., toward directions corresponding to about a half of angles formed by a diagonal line relative to a normal extended from a submerged-nozzle center to individual side lines. Patent Document 2 describes that this method improved the quality of the strands. However, because these methods are assumed for bloom and billet molds, they have gained certain degrees of achievements by supplying the molten metal to both longer and shorter sides. With respect to slabs, there has been remaining an issue that molten metal can hardly be supplied up to the longer-side end face, making it impossible to obtain a sufficient stirring effect of the molten metal.
Patent Documents 3 to 6 propose methods for intending to stir the molten steel within the mold by injecting the molten steel into the mold with a rotatable submerged nozzle while it is rotated.
Patent Document 3 proposes a method for continuously rotating the submerged nozzle at a predetermined rotational speed by a drive device provided outside by rotatably supporting the submerged nozzle via a bearing, providing gaps at a lower end of a tundish nozzle and an upper end portion of the submerged nozzle and introducing inert gas to those gaps so that oxygen in the atmosphere is prevented from being captured into the molten steel through the gaps. As a result, Patent Document 3 describes that a horizontal rotational flow was generated to stir the molten steel within the mold, which improved the quality of strands.
Patent Documents 4 and 5 are improvements of Patent Document 3. Patent Document 4 proposes a method for continuously rotating the nozzle by reaction of the molten steel discharged through discharge holes of the submerged nozzle having circumferentially angled relative to radial directions from a center axis instead of using the drive device, in which the holding-and-rotating mechanism of the submerged-nozzle is identical to that of Patent Document 3. Patent Document 4 describes that the method for stirring the molten steel by rotating the submerged nozzle at a rotational speed corresponding to the flow velocity of the molten steel generated a horizontal rotational flow and stirred the molten steel within the mold to improve the quality of the strands. Further, Patent Document 5 proposes a method for efficiently stirring the molten steel by providing the discharge holes at different heights on the right and the left, injecting the molten steel into the mold at different heights, supporting the submerged nozzle rotatably, and continuously rotating the submerged nozzle at a predetermined rotational speed by a drive device. As a result, Patent Document 5 describes that a rotational flow was generated in horizontal and vertical directions to stir the in-mold molten steel, by which the quality of the strands was improved.
In these cases, there has been a problem that during the flow of the molten steel from the tundish nozzle to the submerged nozzle, pressure reduction occurs at the gap between the tundish nozzle and the submerged nozzle according to Bernoulli's principle, causing large amounts of inert gas to be blown into the molten steel through this gap with the result that large amounts of air bubbles are captured into the strands. On the other hand, although an effect was obtained in terms of molten steel stirring, in this case as well, there has been a problem, for application to slabs, that molten steel can hardly be supplied up to the longer-side end face, so that no effect enough to stir the molten steel can be obtained.
Meanwhile, Patent Document 6 proposes a twin-roll type continuous casting machine in which a flange is provided at the lower portion of the nozzle-extending part, the flange is put into sliding contact with a flange provided at the upper portion of the submerged nozzle, the flanges are pressed to each other by a spring or the like, and the submerged nozzle is continuously rotated at a predetermined rotational speed by providing a drive device. As a result, Patent Document 6 describes that wall shells were prevented from being generated by jetting the hot molten steel derived from the tundish uniformly in the mold so that the molten steel temperature in the mold is made to be uniform to improve the quality of the strands. However, if this method is applied to slab continuous casting machines for iron, there will be a problem of abrasion of the above sliding-contact portion. Although using solid lubricants or the like for ensuring lubrication property is conceivable of, it is not necessarily effective.
Further, in cases where the method for imparting a rotational flow to the molten steel within the mold by continuously rotating discharge directions such as Patent Documents 3 to 6 is applied to slab continuous casting machines, it would be difficult to supply molten steel to both longer side and shorter side parts, and particularly hard to supply molten steel to the longer-side end face, encountering a problem that sufficient stirring effect of the molten steel could not be obtained.
In contrast, Patent Document 7 provides a method for supplying molten steel to the longer-side end face concentratedly and stirring the molten steel smoothly in slab continuous casting machines by installing a submerged nozzle so that discharge directions of the molten steel by a two-hole submerged nozzle are set to between a normal extended from the center axis of the submerged nozzle to the mold shorter side and a diagonal line of the mold. Patent Document 7 describes that a molten steel continuous casting method was provided in which oversupply of discharge flows striking against the longer-side wall surface is eliminated and moreover breakouts are prevented so that ingots of excellent quality can be manufactured and the quality of the strands was improved.
On the occasion of continuous casting, continuing continuous casting with replacing a ladle filled with new molten steel while the molten steel stored in the tundish is taken as a buffer is referred to as sequential continuous castings (which means continuing continuous casting), and the number of ladles of the sequential continuous castings is referred to as number of sequential continuous castings. In this connection, increasing the number of sequential continuous castings is preferable from both energetics and economics points of view. However, the submerged nozzle for continuous casting is always submerged in the molten metal. Further, for ensuring lubricity between the solidified shell of steel and the water-cooled mold, oxide slags which are called as mold powder are formed in the water-cooled mold for continuous casting. Because the submerged nozzle has large dissolved loss at the portions contacting those oxide slags, there has been a problem that the number of sequential continuous castings cannot be increased. This problem is solved by replacing the submerged nozzle with new one as required during sequential continuous castings. The replacement of submerged nozzles in the middle of sequential continuous castings is referred to as quick replacement of submerged nozzles. For example, a quick replacement mechanism for submerged nozzles such as Patent Document 8 is introduced.
Even in such continuous casting machines having the quick replacement mechanism for submerged nozzles, it has been expected to stir the molten metal as required.